Conventionally, there has been a permanent magnet type stepping motor as a structure widely used as an actuator part in such fields as OA equipment, home electrical appliances, automobiles and the like and in many cases, it is provided with an annular stator constructed by combining an exciting coil and a yoke, and a rotor comprising a permanent magnet axially and rotatably supported within a hollow portion of the annular stator. To the peripheral surface of the rotor opposing to the annular stator, a plurality of magnetic poles N and S are magnetized at a constant pitch in an alternative fashion. Further, in the annular stator, a plurality of pole teeth extending to the hollow portion of the stator from the yoke and opposing to the magnetic pole of the rotor are formed at a constant pitch in an annular comb-shape arrangement so that when a pulse signal is supplied to the exciting coil, the yoke including the pole teeth is magnetized and the rotor rotates every predetermined rotational angle by a magnetic action between the pole teeth and the magnetic poles of the rotor. Since this structure has various kinds of advantages such as the capability of achieving a high driving torque in spite of its compactness, attention has been directed to this type of stepping motor of late as a movement of an analogue type indicating meter such as a speedometer and an engine tachometer, for example.
In the case of using this kind of stepping motor as a movement of the analogue type indicating meter, the rotor and the indicator may be connected through a rotary shaft and the rotor interlocked with the indicator may be driven by supplying a pulse signal in response to a measured amount but since the stepping motor operates the rotor stepwise in correspondence to the pitch of the pole teeth, the indication of the measured amount by means of the indicator connected to the rotor also necessarily becomes stepwise, that is, intermittent, so that the rotation of the motor becomes irregular. Accordingly, in the case of using the stepping motor as a movement of the meter, there are proposed, as disclosed in Japanese Unexamined Patent Publication No. 6-38497, a structure in which in order to prevent the rotary shaft of the rotor and the indicator from being directly connected, a reduction gear mechanism comprising a plurality of gears is disposed between them, or as shown in Japanese Unexamined Patent Publication No. 6-82271, a structure in which a pulse signal(voltage) having a stepped waveform closely similar to a sine wave is applied to each of the exciting coils, thereby finely driving the motor stepwise, a so called micro-step drive.
However, in the former structure using the reduction gear mechanism, in addition to the motor itself becoming large-sized, the number of parts is increased to make it difficult to assemble them with ease, so that it is disadvantageous in view of its productivity, and further, there is a fear that the problem of backlash and the like due to the employment of the reduction gear mechanism may arise. On the contrary, in the case of the latter structure using the micro-step drive, it is possible to soften the intermittent motion of the rotor (the indicator) to a level, in which no visual irregularity of the rotor is observed, by finely driving the rotor stepwise according to a predetermined divisional number. However, the rotational angular velocity of the rotor at every step tends to scatter due to an unbalance of the magnetic circuit formed between the magnetic pole of the rotor and the yoke including the pole teeth at every step of the rotor so that it is not sufficient in view of the smoothness of rotation of the rotor.
The present invention has been made by taking these points into consideration and a main object of the invention is to improve the smoothness of rotation of a rotor itself by designing a rotor-yoke structure, and in the case of using the structure as a movement for an analogue type indicating meter, to provide a stepping motor which can secure the rotation of an indicator with no physical disorder without the necessity of providing a reduction gear mechanism.